In the design of electronic article surveillance (EAS) systems which use magnetic type markers, efforts have been made to enhance the uniqueness of the marker's response. One way that this has been accomplished is by increasing the high harmonic content in the voltage pulse generated by the magnetic flux reversal of the marker. As a result, the marker's response signal becomes more easily differentiated and detectable over the lower frequency background noise and magnetic shield noise and signals generated by other magnetic materials often found to exist in EAS systems.
A magnetic marker which exhibits a high degree of uniqueness is disclosed in U.S. Pat. No. 4,660,025, entitled "Article Surveillance Magnetic Marker Having An Hysteresis Loop With Large Barkhausen Discontinuities," which is commonly assigned with the present application. In an embodiment of the invention disclosed in the '025 patent, a marker is formed of an amorphous metal alloy ribbon having locked-in stresses which give rise to large Barkhausen discontinuities in its hysteresis loop. The discontinuities in the hysteresis loop occur at a switching threshold. When the marker is exposed to an alternating interrogation field signal with a peak amplitude that exceeds the switching threshold, high harmonics of the interrogation field signal are generated.
Another magnetic marker which generates high harmonics of an interrogation field signal is disclosed in U.S. Pat. No. 4,980,670, entitled "Deactivatable E.A.S. Marker Having a Step Change In Magnetic Flux." The '670 patent has a common inventor and a common assignee with the present application. The hysteresis characteristic of the marker of the '670 patent exhibits step changes in flux at threshold values of the applied field. In the case of the '670 patent, the desired hysteresis characteristic is brought about by conditioning the material of the marker so that it has a pinned domain wall configuration that remains pinned until the applied field reaches a predetermined threshold value, at which the pinned condition is overcome by the applied field, causing a step change in flux. The step change in flux provides a response signal from the marker which is rich in high harmonic content and is therefore unique and easily detectable.
According to a process disclosed in the '670 patent, a continuous ribbon of amorphous magnetic alloy is cut to form discrete strips of the magnetic alloy material. A magnetic field is applied in the longitudinal direction of the cut strips to form a domain structure, and the resulting domain walls are pinned by annealing. A similar wall-pinning process is described in "Anisotropy Pinning of Domain Walls in a Soft Amorphous Magnetic Material", Schafer et al., IEEE Transactions on Magnetics, Vol. 27, No. 4, July 1991, pp. 3678-3689.
An improved process for making magnetic markers having a step change in magnetic flux is disclosed in U.S. Pat. No. 5,313,192 which is entitled "Deactivatable/Reactivatable Magnetic Marker Having A Step Change In Magnetic Flux", and which has common inventors and a common assignee with the present application.
According to teachings of the '192 patent, it is possible to avoid cumbersome and labor-intensive handling of the cut strips by applying wall pinning processing to a continuous strip of amorphous metal alloy. Regions of the continuous amorphous material are crystallized throughout the bulk of the material at regular intervals along the length of the continuous ribbon. The crystallized bulk regions magnetically isolate the amorphous, pinned-wall intervening regions so that cutting in the crystallized regions to separate the continuous ribbon into individual marker strips does not significantly alter the pinned-wall magnetic properties of the resulting individual markers.
The disclosure of the '025, '670 and '192 patents is incorporated herein by reference.
Although the above-described continuous annealing process of the '192 patent is advantageous in that it permits efficient fabrication of individual markers exhibiting a pinned wall hysteresis characteristic, the crystallized regions provided at regular intervals to magnetically isolate the marker from the adverse effect of cutting the continuous ribbon are somewhat disadvantageous, in that the presence of the crystallized regions at regular intervals predetermine the length of the marker segments. Once the continuous ribbon has been formed with the crystallized regions thereon, the length of the markers to be produced therefrom is fixed. It would be desirable to produce rolls of continuous pinned-wall material from which discrete marker strips of any desired length may be cut.